The present invention relates to a motor drive circuit and in particular to a circuit for controlling low voltage d-c motors.
D-C motors are often used in control applications, for example, to drive a potentiometer. Generally, the d-c voltage is applied to the motor armature through either switch or relay contacts. A problem exists when the d-c voltage being applied to the motor through the relay contacts is small, say, for example, 5 volts. It is well known that such low voltages may not be sufficient to initiate current flow through some relay contacts under poor operating conditions which may occur, for example, when the contacts are dirty. According to one design practice the voltage applied across the relay contacts should be about 25 volts for each set of series-connected contacts in order to provide reasonable assurance that current will flow through the closed contacts. Since 25 volts is much greater than the 5 volts required to run the motor, a voltage divider scheme is required to reduce the voltage applied to the motor.
In one prior art motor drive circuit a 100 volt d-c source is applied to the motor through current limiting resistors and double pole, double throw reversing contacts. As is common in the art, zener diodes along with appropriate blocking diodes can limit the voltage across the motor to the required 5 volts. This prior art circuit results in a very inefficient use of electrical power as about 19 times as much power is dissipated in the current limiting resistors and voltage limiting diodes as in the motor.
It is therefore, the object of my invention to provide an improved circuit that uses relay or switch contacts for applying low d-c voltages to the armature of a d-c motor.